1. Field of the Invention
The invention relates to a process for producing silicon semiconductor wafers with defined defect properties in terms of agglomerated vacancy defects. The invention also relates to silicon semiconductor wafers having these defect properties.
2. Background Art
It is known that single-crystal silicon may have grown-in defects, which are undesirable since they interfere with the functioning of electronic components which are integrated in the silicon, and may even cause functional failure of such devices. Defects of this type are in particular agglomerates of point defects, a distinction being drawn in this context between vacancies and silicon interstitials. If point defects of this type reach supersaturated levels, they tend to collect to form agglomerates. Agglomerated vacancy defects (voids) are often referred to in the literature as COP defects (crystal originated particles), LPDs (light point defects), LLS (localized light scatterers), LSTD (laser scanning tomography defects) or FPD (flow pattern defects). Silicon interstitials are referred to as “A defects” (A-swirl-defects) or Lpit's (large etch pits), if the agglomerates, on account of their size, are already forming secondary defects in the form of dislocations, and “B defects” if secondary defects of this type have not yet occurred.
It is also known that the point defects are formed during the production of the single crystal, and in this context the ratio V/G of the pulling rate V and the axial temperature gradient G at the growth front of the crystal are the defining parameters. If the ratio V/G is above a critical value ξc, which nowadays is currently assumed to be approximately 0.134 mm2min−1K−1, point defects of the vacancy type dominate, whereas at a ratio V/G below the critical value ξc, silicon interstitials dominate. During cooling of the single crystal, the point defects which have formed reach supersaturated levels and can accumulate to form larger groupings. It is known that the size of the agglomerated vacancy defects is influenced to a crucial extent by the cooling rate q at which the single crystal is cooled from approximately 1100° C. to lower temperatures, specifically in such a manner that the defect size decreases with increasing cooling rate, whereas the density of these defects rises. Since the cooling rate q is approximately directly proportional to the axial temperature gradient G, it is therefore also possible to influence the size of the agglomerated point defects by suitably controlling the ratio V/G during pulling of the single crystal, and if the axial temperature gradient G is known, it is possible to draw conclusions as to the size of the agglomerated vacancy defects which form. Agglomerated vacancy defects are generally octahedral in form. Details on the size of defects of this type describe a volume of a sphere corresponding to the defect volume.
The radiant heat emitted from the edge of the single crystal causes the axial temperature gradient G to rise toward the edge of the single crystal. A direct consequence of this radial dependency of the axial temperature gradient G(r) on the radial position r of the single crystal and of the abovementioned dependency of the size of the agglomerated vacancy defects on the axial temperature gradient G is that the size of these defects is not constant in the vacancy zone of a semiconductor wafer, but rather decreases from the center toward the edge of the semiconductor wafer, whereas the density of the defects increases in the same direction.
The axial temperature gradient G can be calculated accurately using computer codes (for example FEMAG from FEMAGSoft S.A., Belgium), which means that it is possible to control the ratio V/G by controlling the pulling rate. At the same time, given knowledge of the axial temperature gradient G, the radial distribution of the incorporated vacancies and therefore in turn the radial variation in the mean size of the agglomerated vacancy defects can be calculated. Formulae which were developed by Voronkov (Voronkov, V. V. and Falster, R. (1999) J. APPL. PHYS., 86, (11) 5975 and Voronkov, V. V. and Falster, R. (1998) J. CRYSTAL GROWTH, 194 76) form the theoretical basis for these calculations. These formulae allow, for example, experimentally verifiable predictions to be made as to the concentration of incorporated vacancies, the concentration of agglomerated vacancies and the size of these agglomerates.
The presence of agglomerated vacancy defects start to become a problem, given an increasing integration density of electronic components and the associated reduction in size of the structures used to form them, if the size of these defects is in the region of the feature size of the components.
To solve this problem, it has already been proposed that the ratio V/G be controlled in such a manner that no agglomerated point defects are formed, since the supersaturation of the point defects which this requires is not reached. However, control of this type, in particular when producing silicon semiconductor wafers with large diameters of 200 mm and above, can only be realized with considerable difficulties, since on account of the dependency of the axial temperature gradient G on the radial position r, only a narrow process window within which V/G may vary is available. As demonstrated by an accurate calculation based on the Voronkov formulae, the V/G range in which there is no agglomeration of vacancies in the vacancy-rich zones is extremely narrow and therefore not technically accessible. Similar considerations apply to the B defects on the Si interstitial-rich side. Only the formation of Lpit's can be avoided by a greater V/G range, since for this purpose a critical size of the interstitial aggregates has to be exceeded.
Another way of solving the problem consists in dissolving agglomerated vacancy defects by means of a heat treatment of the semiconductor wafers in a region close to the surface. A heat treatment of this type is likewise complex and, in particular if it is carried out in an oxidizing atmosphere, cannot be fully realized (Ji Wook Seo and Young Kwan Kim, JOURNAL OF THE ELECTROCHEMICAL SOCIETY, 149 (7) G379-G383 (2002)). According to current opinion as expressed in the prior art, a two-stage mechanism is active during the heat treatment; in the first stage, an oxide layer is removed from the inner surface of the agglomerated vacancy defects. This requires the oxygen to diffuse out of the region close to the surface, thereby eliminating the oxygen supersaturation which is present there. Only after that can the agglomerated vacancy defects be dissolved by recombination with silicon interstitials and by outdiffusion of vacancies. Although the production of a thermal oxide layer on the semiconductor wafer promotes the formation of silicon interstitials, an oxidizing heat treatment of the semiconductor wafers aimed at achieving this is considered unfavorable compared to a heat treatment in a nonoxidizing atmosphere, such as argon, since it impedes the elimination of the oxide layer from the inner surface of the agglomerated vacancy defects which is required in the first step.